You may want to try the Perelli MT43 (DOT trials tire), I was surprised how well it held up to all the lava/cinder/asphalt. I had about 2,100 miles on it when I started my Big Island ride and when I was pau it had about 2,900 miles and about 1/8" center tread left with no missing lugs. It made it through South Point, Road to the Sea, Mana Road and the other road above Mana without any problems. BTW, I had it on a WR290R.

I have to second the recommendation on the Pirelli MT43. The Dunlop 803 is made for a 148 pound trials bike, whereas the MT43 is purpose built for a full size bike. You won't get the same traction as a true trials tire (rubber compound is a little harder) but you'll still get better traction than a knobby (outside of mud) and a lot better tire life than a pure trials tire. The sidewall is stiff too to reduce the chance of pinch flats on heavier 4-stroke bikes (250s and 450s). I don't think you'll regret it. I keep the 803 on my 300 Raga and the MT43 on my WR450.

I know I don't live in Hawaii any longer but I still love following the ride reports of the Big Island crew!